Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Abstract

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, K max. A superposition model is developed to predict simultaneously the effects of stress intensity range ( ΔK) and K ratio ( K min/ K max). The effect of K max is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip.